Isolated power supplies, particularly primary side switched power supplies, typically required feedback of a metric of an electrical characteristic from a secondary side to a primary side. The electrical characteristic may be an output voltage, an output voltage error, an output current or an output current error without limitation. Feedback of an output electrical characteristic according to the prior art may be accomplished by any of a plurality of methods. In a first method, known as a coupled winding feedback, the secondary side electrical characteristic is reflected to the primary side by the turns ratio of the power transformer. In case where high voltage isolation is required, the coupled winding feedback presents a conflict between the need for close coupling, required for feedback, and the need for high isolation voltage. In a second method, a feedback or bias winding is added to the power transformer, the feedback or bias winding voltage reflecting the secondary side electrical characteristic to be controlled. Close coupling between the feedback or bias winding and the secondary winding is required, and in high power cases cross-regulation limits the load-regulation range. Thus, neither of the above solutions is suitable for high power, high voltage situations.
In a third method an opto-isolator or transformer coupler feedback is provided to pass information regarding the electrical characteristic metric from the secondary side to the primary side. Opto-isolators waste energy, and in addition their transfer characteristics vary, inter-alia, with age and temperature. Transformer coupled feedback is stable, however requires additional modulating and demodulating circuitry since transformers are unable to pass DC voltage or current in the absence of additional circuitry.
What is desired, and is not provided by the prior art, is a method of providing feedback of an electrical characteristic from a secondary side to a primary side, suitable for use with high voltage and high loads without requiring an additional transformer with associated electronics.